San Jose State University SJSU ScholarWorks Faculty Publications Physics and Astronomy 1-1-2013 The densest galaxy J. Strader Michigan State University A. C. Seth University of Utah J. P. Brodie University of California Observatories D. A. Forbes Swinburne University G. Fabbiano Harvard-Smithsonian Center for Astrophysics See next page for additional authors Follow this and additional works at: https://scholarworks.sjsu.edu/physics_astron_pub Part of the Astrophysics and Astronomy Commons Recommended Citation J. Strader, A. C. Seth, J. P. Brodie, D. A. Forbes, G. Fabbiano, Aaron J. Romanowsky, C. Conroy, N. Caldwell, V. Pota, C. Usher, and J. A. Arnold. "The densest galaxy" Astrophysical Journal Letters (2013). https://doi.org/10.1088/2041-8205/775/1/L6 This Article is brought to you for free and open access by the Physics and Astronomy at SJSU ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SJSU ScholarWorks. For more information, please contact [email protected]. Authors J. Strader, A. C. Seth, J. P. Brodie, D. A. Forbes, G. Fabbiano, Aaron J. Romanowsky, C. Conroy, N. Caldwell, V. Pota, C. Usher, and J. A. Arnold This article is available at SJSU ScholarWorks: https://scholarworks.sjsu.edu/physics_astron_pub/21 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 doi:10.1088/2041-8205/775/1/L6 ©C 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE DENSEST GALAXY Jay Strader1, Anil C. Seth2, Duncan A. Forbes3, Giuseppina Fabbiano4, Aaron J. Romanowsky5,6, Jean P. Brodie6 , Charlie Conroy7, Nelson Caldwell4, Vincenzo Pota3, Christopher Usher3, and Jacob A. Arnold6 1 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA; [email protected] 2 University of Utah, Salt Lake City, UT 84112, USA 3 Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn, VIC 3122, Australia 4 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA 5 Department of Physics and Astronomy, San Jose´ State University, San Jose, CA 95192, USA 6 University of California Observatories/Lick Observatory, Santa Cruz, CA 95064, USA 7 Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA Received 2013 July 26; accepted 2013 August 14; published 2013 August 30 ABSTRACT We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo elliptical galaxy 8 NGC 4649 (M60), which we call M60-UCD1. With a dynamical mass of 2.0 × 10 M0 but a half-light radius of only ∼24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local universe. It has a two-component structure well fit by a sum of S ersic´ functions, with an elliptical, compact (rh = 14 pc; n ∼ 3.3) inner component and a round, exponential, extended 38 −1 (rh = 49 pc) outer component. Chandra data reveal a variable central X-ray source with LX ∼ 10 erg s that could be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old (210 Gyr) and of solar metallicity, with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light-element self-enrichment; such self-enrichment may be generically present in dense stellar systems. The velocity dispersion (σ ∼ 70 km s−1) and resulting dynamical mass-to-light ratio (M/LV = 4.9 ± 0.7) are consistent with—but slightly higher than—expectations for an old, metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this M/LV . The stellar density of the galaxy is so high that no dynamical signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with MB ∼−18 to −19. Key words: galaxies: dwarf – galaxies: elliptical and lenticular, cD – galaxies: individual (M60) – galaxies: kinematics and dynamics – galaxies: star clusters: general Online-only material: color figures 1. INTRODUCTION There is more at stake than the natural desire to understand these novel stellar systems. If a significant fraction of UCDs Objects with sizes and masses between those of globular contain dark matter, then they form a populous class that clusters and compact ellipticals (rh ∼ 10–100 pc; MV ∼−9to must be included in counts of subhalos for comparisons to −14) were first discovered in spectroscopic surveys of galaxy cosmological theory. Further, if some UCDs are formed by tidal clusters (Hilker et al. 1999; Drinkwater et al. 2000). They were stripping, their chemical and structural properties help trace quickly dubbed “ultra-compact dwarf ” galaxies (UCDs), even galaxy transformation. though their galaxian nature was unclear. Large populations of Here we report the discovery of an extraordinary UCD around UCDs have been discovered in Fornax, Virgo, and other galaxy the Virgo elliptical NGC 4649 (M60). It has a half-light radius 8 clusters, as well as in group and field environments—see reviews of 24 pc but a stellar mass of 2 × 10 M0,givingitthe in Chilingarian et al. (2011), Norris & Kannappan (2011), and highest surface density of any galaxy in the local universe. Brodie et al. (2011). We also present evidence that this UCD may contain a central UCD formation scenarios have coalesced around two poles: supermassive black hole. star cluster or galaxy. In the former scenario, UCDs form the massive end of the normal sequence of globular clusters (Mieske 2. DATA et al. 2012). Further, if some star clusters form in gravitationally 2.1. Imaging bound complexes, these can merge to make objects that are larger and more massive than single clusters (Br uns¨ et al. 2011). We discovered M60-UCD1 in the Hubble Space Telescope Alternatively, UCDs could be galaxies that formed in individ­ (HST)/Advanced Camera for Surveys (ACS) imaging of Strader ual dark matter halos—either “in situ,” as unusual, extremely et al. (2012). We have a single orbit of imaging split between compact galaxies—or as the products of tidal stripping of more F 475W and F 850LP (hereafter g and z). M60-UCD1 is massive progenitor galaxies (e.g., Drinkwater et al. 2003). located at (R.A., decl.) = (190.8999, 11.5347) in decimal J2000 A reasonable synthesis of these scenarios may be that the coordinates. This is at a projected distance of only ∼6.6 kpc from 6 least-massive “UCDs,” with ∼10 M0, are largely star clusters, the center of M60 (Figure 1; assuming a distance of 16.5 Mpc; 8 while the most massive objects (210 M0) are galaxies, or the Blakeslee et al. 2009). No mention is made of M60-UCD1 in tidally stripped remnants thereof. At intermediate masses both previous Virgo surveys, including the ACS Virgo Cluster Survey star clusters and galaxies may coexist (e.g., Norris & Kannappan (C otˆ eetal.´ 2004). It is present in the Sloan Digital Sky Survey 2011; Brodie et al. 2011). Data Release 7 photometric catalog (Abazajian et al. 2009) 1 The Astrophysical Journal Letters, 775:L6 (6pp), 2013 September 20 Strader et al. Figure 1. HST/ACS color image of the central region of M60, showing the location of M60-UCD1 (solid circle). A typical UCD (A32, ∼3 × 106 M0; Strader et al. 2012) is also marked (dashed circle) for reference. (A color version of this figure is available in the online journal.) as J124335.96+113204.6, and was classified by Simard et al. empirical point-spread function (PSF) using a custom software (2011) as a background galaxy. package as described in Seth et al. (2006). The PSF (10× subsampled) was derived from point sources in the images. The 2.2. Spectroscopy fits were performed on a 511×511 image centered on the UCD, A spectrum of M60-UCD1 was obtained on the night of 2012 with the background galaxy gradient from M60 itself removed. January 17 with Keck/DEIMOS (Faber et al. 2003), utilizing the The fits are not very sensitive to the fitting box size. 1200 l/mm grating centered at 7800 Å and a 111 slit (resolution From the residual map (Figure 2), it is clear that a single ∼1.5 Å). We obtained three 30 minute exposures in 011.8 seeing. Sersic´ component provides a poor fit. In particular, the ellipticity Using the spec2d pipeline (Cooper et al. 2012), the spectra were of M60-UCD1 becomes more circular at larger radii, leaving a extracted, calibrated, and combined in the standard manner to residual along the minor axis. The radial shape of the surface produce a final one-dimensional spectrum. brightness profile is also poorly fit. To improve stellar population constraints, further spec­ However, using a two-component Sersic´ model, a very good 2 = troscopy was undertaken with MMT/Hectospec (Fabricant et al. fit (χν 1.07 in g) is obtained. Table 1 gives the parameters for 2005) on 2012 May 16, using the 270 l/mm grating with wave­ the single and double Sersic´ g-band fits and the double Sersic´ z length coverage from 3700 to 9100 Å and 5 Å resolution. Three fits. For the two-component fits the S ersic´ parameters n and re 20 minute exposures were taken in 011.9 seeing. These Hectospec are very similar between the filters. Because the g band provides data were pipeline-reduced in a standard manner as described a much better fit (probably due to PSF modeling issues in z), all in Mink et al. (2007). structural values cited are from the g fits.